A conventional audio amplifier system (100) is illustrated in FIG. 1. The audio amplifier system (100) includes three operation amplifiers (AMP10–AMP12), two signal-coupling capacitors (CIN1–CIN2), six resistors (R11–R16), and two speakers (SPK1–SPK2).
Resistors R15 and R16 are series connected between VHI and GND, forming a resistor divider that produces a reference voltage (VREF). The reference voltage is coupled to the non-inverting inputs of amplifiers AMP10–AMP12. Amplifier AMP 11 is arranged to operate as a buffer such that a buffered version of the reference voltage is provided to node OUT3.
The reference voltage (VREF) is necessary to bias the amplifiers in their optimal common mode range, such that a maximum voltage swing is available between the amplifiers output nodes. Since the amplifiers are biased into their optimal performance range by the reference voltage (VREF), it is necessary isolate the DC level of the input signal from the amplifier. Signal-coupling capacitors CIN1 and CIN2 are arranged to provide DC isolation between the input signals and the biasing condition for the amplifiers.
Amplifier AMP10, resistor R11, and resistor R12 are arranged to operate as an inverting amplifier (X1). Inverting amplifier X1 receives audio input signal IN1 via signal coupling capacitor CIN1. The gain associated with inverting amplifier X1 is determined by resistors R11 and R12. An output signal is provided by inverting amplifier X1 at node OUT1 in response to input signal IN1. Inverting amplifier X1 drives speaker SPK1 between nodes OUT1 and OUT3.
Amplifier AMP12, resistor R13, and resistor R14 are arranged to operate as another inverting amplifier (X2). Inverting amplifier X2 receives audio input signal IN2 via signal-coupling capacitor CIN2. The gain associated with inverting amplifier X2 is determined by resistors R13 and R14. An output signal is provided by inverting amplifier X2 at node OUT2 in response to input signal IN2. Inverting amplifier X2 drives speaker SPK2 between nodes OUT2 and OUT3.